Harnessing Dielectric Confinement on Tin Perovskites to Achieve

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Harnessing Dielectric Confinement on Tin Perovskites to Achieve Emission Quantum Yield up to 21% Jin-Tai Lin, Chen-Cheng Liao, Chia-Shuo Hsu, Deng-Gao Chen, Hao Ming Chen, Ming-Kang Tsai, Pi-Tai Chou, and Ching-Wen Chiu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03148 • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 6, 2019

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Journal of the American Chemical Society

Harnessing Dielectric Confinement on Tin Perovskites to Achieve Emission Quantum Yield up to 21% Jin-Tai Lin,† Chen-Cheng Liao,‡ Chia-Shuo Hsu,† Deng-Gao Chen,† Hao-Ming Chen,†,* MingKang Tsai,‡,* Pi-Tai Chou,†,§,* Ching-Wen Chiu†,* †Department

of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei, 10617 Taiwan

‡Department

of Chemistry, National Taiwan Normal University, No. 88, Section 4, Ting-Zhou Road, Taipei 11677, Taiwan §Center for Emerging Materials and Advanced Devices, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei, 10617 Taiwan *e-mail: [email protected]; [email protected]; [email protected]; [email protected] ABSTRACT: Tin perovskite nanomaterial is one of the promising candidates to replace organic lead halide perovskites in lighting application. Unfortunately, the performance of tin-based system is markedly inferior to those featuring toxic Pb salts. In an effort to improve the emission quantum efficiency of nanoscale 2D layered tin iodide perovskites through finetuning the electronic property of organic ammonium salts, we came to unveil the relationship between dielectric confinement and the photoluminescent properties of tin-iodide perovskite nano-disks. Our results show that increasing dielectric contrast for organic versus inorganic layers leads to a bathochromic shift in emission peak wavelength, a decrease of exciton recombination time, and importantly a significant boost in the emission efficiency. Under optimized conditions, a leap in emission quantum yield to a record high 21% was accomplished for the nanoscale thienylethyl ammonium tin-iodide perovskite (TEA2SnI4). The as prepared TEA2SnI4 also possessed superior photostability, showing no sign of degradation under continuous irradiation (10 mW/cm2) over a period of 120 hours.

INTRODUCTION Emerging emissive nanomaterials have received considerable attention due to their unique photophysical properties such as enlarged exciton binding energy and enhanced light absorption cross section with respect to the corresponding bulk materials.1 Among these nanomaterials, lead halide perovskite (CsPbX3 where X = Cl, Br, I) nanocrystals have shown excellent photoluminescence performance, including high photoluminescence quantum yield (PLQY), narrow full width at half maximum (FWHM) and tunable emission covering the visible range.2-11 As a result, lead halide perovskite nanocrystals have been widely studied for their potential applications in light emitting diodes12-15 and laser.16-18 Despite these vastly developed research fields, the toxicity remains to be a major obstacle for commercializing lead halide perovskite-based innovations. As for the replacement, the demand for nontoxic element-based perovskite materials continues to grow in the past few years.19,20 Among these nontoxic alternatives, divalent tin cation has been considered as a good candidate in replacing Pb2+ and the potential application of tin-halide perovskites in optoelectronic devices has also been investigated.21-30 However, Sn2+ undergoes facile oxidation to its tetravalent state, creating a high defect density in the perovskite lattice. These defects would generate trap states, leading to rapid non-radiative

relaxation of excitons. It is thus much more challenging to achieve high PLQY and stability with tin perovskites.31,32 Although the high PLQY (88-95%) can be accomplished by the self-trapped state emission of tin layered perovskites,33,34 the broad emission (~135 nm FWHM) may not be suitable for display application that requires high color purity. Up to current stage the highest PLQY value of direct band narrow emissions of tin-based 2D perovskite nanomaterials can only reach 6.4%,35 which is still far inferior to the lead-based perovskite nanomaterials (~100%).4,36 Both experimental and theoretical studies on quantum dots have shown that the emission intensity is positively correlated to their exciton binding energy (Eb), which can be fine-tuned by quantum confinement. For example, improved PLQY of AlGaN quantum well could be realized by increasing the exciton binding energy through reducing the width of quantum well.37 Similar phenomenon also has been observed in the luminescent perovskite materials. Pullerits reported that high PLQY of lead perovskite nanoparticles can be attributed to their exciton binding energy that is approximately 5 times higher than that of bulk crystals.38 Likewise, nanoscale 2D layered tin perovskites35,39,40 always exhibited higher PLQY than their corresponding 3D nanocrystals41 due to the increased Eb of the 2D structure.42 Therefore, raising the exciton binding energy could be an effective strategy

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to further enhance the emission intensity of tin perovskite-based nanomaterials. Unfortunately, the exciton binding energy of mono-layered 2D tin perovskites cannot be further increased by reducing the lattice dimension, because the quantum confinement effect has reached the maximum along the packing direction. In an aim to boost emission efficiency one must resort to a different strategy in enhancing Eb of the nanoscale 2D layered tin perovskites. Studies of emissive quantum dots also demonstrate that exciton binding energy of semiconducting materials, in addition to quantum confinement, can be harnessed by dielectric confinement effect. Dielectric confinement is caused by the mismatch of dielectric constants between semiconductor and its surrounding. During the synthesis of nanoparticles, organic surfactants are applied to control the topology of nanoparticles and prevent aggregation. The surrounding organic layer with low dielectric constant is less polarizable and hence decreases the screening of the hole-electron Coulomb interaction, resulting in an increase of the exciton binding energy. For the 2D perovskites, we also anticipate that the discrepancy of dielectric constants between the inorganic framework (semiconductor) and the organic layers (surrounding) should give rise to the dielectric confinement imposed on the exciton, which can be modulated by changing the composition of the organic cations.43 Theoretically, the correlation among Eb of 2D, 3D excitons and dielectric constant of media is expressed by the following equation:44 𝜀𝑤 2

( )𝐸

𝐸2𝐷 𝑏 =4

𝜀𝑏

3𝐷 𝑏

(1)

where εw and εb are the dielectric constants of the inorganic frameworks (the well layer) and the organic cations (the barrier layer), respectively. 𝐸3𝐷 𝑏 stands for the exciton binding energy of the corresponding 3D perovskite. According to eq. (1), the decrease in εb and/or the increase in εw would lead to an increase in the 2D exciton binding energy. The emission quantum efficiency of a layered tin perovskite may thus be further improved by enlarging the dielectric contrast between the tin-halide layer and the intercalating ammonium cation. The influence of dielectric confinement on the luminescent nanomaterials is not only limited to the exciton binding energy. Also, the optical band gap would be affected. Under the same composition and width of quantum well, Takagahara has pointed out that a semiconductor nanomaterial surrounded by a medium with a small dielectric constant exhibits an optical band gap in Rydberg unit given by:45 𝐸𝑔 = 𝐸𝑜 ―

0.248ε𝑤 ε𝑏

(2)

where Eo is the band gap of semiconductor in Rydberg unit without consideration of dielectric confinement effect. The second term of eq. (2) is derived from the direct Coulomb interaction between an electron and a hole, leading to a decrease of band gap. In short, we expected that tin perovskites with a stronger and redshifted emission could be obtained upon enhancing the

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dielectric confinement on the 2D layer structure. If successful, this, together with further optimizing the additive effect of aliphatic acids,35 might serve as an effective approach to attain highly emissive and stable nanoscale 2D layered tin perovskites. To prove the concept, herein, we investigated a series of organic ammonium cations to fine-tune the dielectric confinement in 2D tin-iodide perovskites. As a result, a correlation of increasing strength of dielectric confinement resulting in the increase of PLQY has been established. Among various ammonium cations incorporated nanoscale 2D layered tin perovskites, TEA2SnI4 (TEA: thienylethyl ammonium) showed the largest εw/εb ratio and hence the highest PLQY. Further optimization of TEA2SnI4 by appropriate aliphatic acid additives achieved the record-high emission quantum yield up to 21% and long-term stability that revealed no sign of degradation after continuous irradiation for 120 hours. Details of the experimental results, theoretical approaches and discussion are elaborated in the following sections. RESULTS AND DISCUSSION Prior to the systematic study of dielectric confinement on the optical properties of tin perovskites, proper selection of ammonium cations is indispensable. Also, the packing defect and structural variation could potentially be the dominant factors governing the photo-physical properties of the perovskite nanomaterials. As reported in our previous study,35 2D layered tin perovskite nano-disks prepared from aliphatic ammonium cations all exhibited poor morphology due to the lack of interlayer interactions. These structural defects resulted in not only poor stability of the nano-disk, but also low PLQY values (< 0.5%).35 To avoid the defect dominated optical properties that interfere in our mechanistic approach, aliphatic ammonium cations were excluded. Also eliminated are bulky 2-aryl ethyl types ammonium cations because of the undesired distortion of the inorganic framework. For instance, tin perovskite nano-disks incorporating 2naphthalenyl ethyl ammonium (NEA) relevant cations that are large in sizes yielded a weak and multicomponent luminescence with PLQY of ~0.03% (see Figure S1). Prudentially, we decided to focus on the derivatives of phenyl ethyl ammonium (PEA) cations, which do not notably disturb the bonding situation around the Sn center.31,41 These include (parafluorophenyl)ethyl ammonium (p-FPEA), (parachlorophenyl)ethyl ammonium (p-ClPEA) and (parabromophenyl)ethyl ammonium (p-BrPEA). Those PEA derivatives with substitution at the ortho-position were also discarded due to their strong perturbation to the [SnI4]2- layer. For example, the tin perovskite obtained with ortho-fluoro PEA (o-FPEA) exhibited an absorption peak that was 21 nm blue shifted compared to that using PEA. The observed blue shift can be rationalized by the Sn-I-Sn bond angle deformation from 156.5o in PEA2SnI4 to 153.3o in o-FPEA2SnI4.46 Such a structural deformation could be minimized by replacing o-FPEA with para-fluoro PEA (p-FPEA).46 We also selected the PEA derivative with


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Journal of the American Chemical Society thiophene moiety to replace the phenyl group, forming 2(thiophen-2-yl)ethyl ammonium (TEA) (Figure 1a). As revealed in Figure S2, the observed Sn-I-Sn bond angle of 156 o and Sn-I bond lengths range between 3.12 Å to 3.14 Å in the TEA containing perovskite (TEA2SnI4), which are comparable to that of PEA2SnI4 (vide infra). After prescreening the suitable organic ammonium cations, PEA2SnI4, p-FPEA2SnI4, p-ClPEA2SnI4, p-BrPEA2SnI4 and TEA2SnI4 were prepared and investigated (see SI for details of synthetic routes) To determine the structures of the as-prepared nanodisks, X-ray powder diffraction (XRD) analysis was carried out. All of the synthesized tin-iodide perovskite nanodisks are characteristic of a set of strong 00l reflections, corresponding to a periodic stacking sequence constructed by inorganic framework and organic layers, as illustrated in XRD patterns (see Figure 1b). Notably, the interlayer spacing of (00l) facets in perovskite nanodisks (i.e., the spacing among the perovskite layers) seems to be dominated by the size of the organic counter cations, among which the TEA2SnI4 nano-disks reveal the smallest interlayer spacing of 1.563 nm. Figure S3 shows the transmission electron microscope (TEM) images of these tin-iodide perovskite nano-disks, which, independent of the different ammonium cations, all reveal squared, uniformed 2D structures as well as significant contrast arising from multi-layer stacking, manifesting a longrange stacking alone (00l) direction. The atomic force microscope (AFM) images of tin-iodide perovskite nanodisks (see Figure S4a) illustrate that the thickness of these nano-disks is approximately 15-18 nm and independent of the species of incorporating organic cations. The results of AFM indicate that each nano-disk contains about 10 layers of inorganic framework, confirming the long-range stacking of perovskite.

properties of the tin perovskite nano-disks could be systematically probed. The absorption and photoluminescence spectra of the various as-prepared tin-iodide perovskite nano-disks are shown in Figure 2, and the pertinent PLQY, FWHM, absorption and emission peak wavelengths are summarized in Table 1. The results reveal that the photophysical properties of perovskite nano-disks are indeed influenced by the intercalated organic cations. The corresponding emission peak wavelength is red shifted in the order of 635.1 nm (p-ClPEA2SnI4) < 637.2 nm (pBrPEA2SnI4) < 638.0 nm (PEA2SnI4) < 640.1 nm (pFPEA2SnI4) < 645.2 nm (TEA2SnI4). Also, the absorption peak wavelength reveals the similar trend (618.5 nm for pClPEA2SnI4 < 619.4 nm for p-BrPEA2SnI4 < 620.0 nm for PEA2SnI4 < 621.2 nm for p-FPEA2SnI4 < 624 nm for TEA2SnI4). In addition, the observed lifetime (obs) increases as increasing the emission peak wavelength (Figure S5 and Table S1), which is also accompanied by the increase of PLQY in the order of p-ClPEA2SnI4 (54.6 ns, 1.26%) < p-BrPEA2SnI4 (63.3 ns, 1.73%) < PEA2SnI4 (94.0 ns, 2.70%) < p-FPEA2SnI4 (110.1 ns, 3.67%) < TEA2SnI4 (139.5 ns, 5.80%) for the pristine (without modification by the acid additives, vide infra) tin-iodide perovskite nano-disks. According to PLQY = kr/(kr + knr) = kr × obs where kr and knr denote the radiative decay (kr = 1/r, r: radiative lifetime) and non-radiative decay rate constants, respectively, kr and knr can thus be deduced and listed in Table S1. Interestingly, both kr and knr reveal a good correlation with respect to the emission peak wavelength, in which kr increases and knr decreases upon further red shifting the peak wavelength. Importantly, the increase of radiative decay rate constant kr implies the increase of binding energy, which should be relevant to the changes of dielectric confinement in these cation-dependent 2Dtin perovskite nano-disks, as derived previously and discussed in the following section. Also, the decrease of knr, hence suppression of the non-radiative decay pathways, can be rationalized by the increase of binding energy that may cause the reduction of the packing defects.

Figure 1. a) Chemical structures of p-ClPEA, p-BrPEA, pFPEA, PEA, and TEA. b) XRD patterns for tin-iodide perovskite and the corresponding d-spacing alone the (00l) facet.

Because the distortion of inorganic framework has been minimized (vide supra), we assume that any observed spectral changes for the tin perovskite nano-disks in this study should be mainly arisen from the modulation of the electronic contrast between inorganic layers and organic cations. Standing on this hypothesis, the relationship between dielectric confinement and photophysical

Figure 2. Absorption photoluminescence spectra of tin perovskite nano-disks in toluene. ex = 375 nm. The inset pictures show photos of the cell containing PEA2SnI4 (left),



p-FPEA2SnI4 (center), TEA2SnI4 (right) nano-disks prepared with the optimized condition under illumination with 375 nm laser diode (10 mW/cm2).

Table 1. PLQY values, emission peak wavelengths, FWHM, and radiative lifetime of pristine tin-iodide perovskite nano-disks in toluene. Herein, all of the values are obtained via averaging the results of five individual samples. PLQY (%)

PL peak (nm)

Absorpti on peak (nm)

FWHM (nm)

Without aliphatic acid additives TEA SnI 2

5.80 ± 0.25

4

p-FPEA SnI

3.67 ± 0.14

PEA SnI

2.70 ± 0.28

2

4

2

4

645.2 ± 1.2 640.1 ± 0.4 638.0 ± 1.2

624.0 ± 0.2 621.2 ± 0.4 620.0 ± 0.4

pBrPEA SnI

1.73 ± 0.23

637.2 ± 1.2

619.4 ± 0.6

pClPEA SnI

1.26 ± 0.08

635.1 ± 0.8

618.5 ± 0.6

2

4

2

4

TEA SnI 2

a

4

pFPEA SnI 2

4

c

31.8 ± 0.1

With aliphatic acid additives 18.85 ± 2.17 9.94 ± 1.23

4

PEA SnI 2

b

32.4 ± 0.1 30.8 ± 0.1 33.6 ± 0.2 30.4 ± 0.1

6.40 ± 0.42

645.2 ± 1.2 640.1 ± 0.4 638.0 ± 1.2

624.0 ± 0.2 621.2 ± 0.4 620.0 ± 0.4

32.4 ± 0.1 30.8 ± 0.1 33.6 ± 0.2

Radiativ e lifetime (ns) 2.41 ± 0.03 3.00 ± 0.04 3.48 ± 0.11 3.66 ± 0.18 4.33 ± 0.25 1.86 ± 0.02 2.79 ± 0.03 3.43 ± 0.09

a With

0.1 vol% pentanoic acid additive. b With 1.5 vol% 4-methylpentanoic acid additive. c With 2 vol% 3-methylbutyric acid additive.

As mentioned above, the optoelectronic properties of the organic-inorganic hybrid nanomaterials would be sensitive towards the nature of the organic/inorganic fragments and the lattice defect of the material. To observe the organic cation induced dielectric confinement effect, we must lower the defect density of perovskite nano-disks. Therefore, we decided to further optimize the synthesis condition through the introduction of aliphatic carboxylic acid additive. As demonstrated in our previous work, the PLQY of PEA2SnI4 could be improved from 2.7% to 6.4% by adding 2 vol% of 3-methylbutyric acid into the anti-solvent. The significant elevation in PLQY implies the successful suppression of defects during the nano-disk nucleation process.35 Notably, the addition of aliphatic acid does not affect neither the structural parameters nor the emission wavelengths of the tin perovskite nano-disks (Table 1). The XRD pattern reveals that the layered structure of TEA2SnI4 remains unchanged in the presence of pentanoic acid additive, ruling out the possibility that the improvement of emission is arisen from the structural variation (Figure S6). Also, the AFM images of tin perovskite nano-disks indicate that the thickness of nanodisks is approximate 15 nm for both samples under synthetic condition with or without additive (Figure S4). The independence of the optical band gap of tin perovskite nano-disks on the acid additives suggests that the contribution of aliphatic acid additives to the dielectric confinement is negligible. However, an

impressive enhancement in the emission quantum yield of p-FPEA2SnI4 (from 3.67% to 9.94%) and TEA2SnI4 (from 5.80% to 18.85%) was accomplished via the inclusion of 1.5 vol% of 4-methylpentanoic acid and 0.1 vol% of pentanoic acid, respectively. Details of the acid additive optimization are summarized in Table S2 and Table S3 of the Supporting Information. The increase of observed lifetime of the nano-disks correlates well with the improvement of PLQY. The emission lifetime of the TEA2SnI4 nano-disks increased from 139 ps to 348 ps, accompanied by a minor, long-lived component that decayed with a lifetime of 5 ns (see Figure S5 and Table S1 of Supporting Information). More importantly, the decrease of knr value of nano-disks illustrates the successful suppression of lattice defect (as shown in Table S1). Therefore, it would be more appropriate to use the optical properties of the optimized materials for the discussion of the dielectric confinement on tin perovskite nano-disks elaborated hereafter. To verify that the observed photophysical properties are associated with the dielectric confinement effect, we then performed density functional theory (DFT) simulations for the studied tin-iodide perovskite nanodisks. The dielectric constants of well and barrier layers were estimated using a neutralized ghost-layer model under the independent particle approximation. Detail of the computational methodology is elaborated in SI. As a result, the dielectric constants of the organic layers, i.e. 𝜀𝑏 of the barrier layers, only changed minimally due to the localized electronic density of these ammonium cations and flexible intermolecular packing to release the steric repulsion (see Table S4). Interestingly, the calculated dielectric constant of inorganic layers (𝜀𝑤 of the well layers) changed noticeably. The schematic representation of the predicted dielectric constants for TEA2SnI4, pFPEA2SnI4 and PEA2SnI4 were summarized in Figure 3. The calculated 𝜀𝑤 values seemed to increase in respect to the polarizability of the [SnI4]2- sheet. The TEA cation containing sulfur-fused five-member ring was considered to introduce more different intermolecular packing in the barrier layer than those of PEA series, despite that 𝜀𝑏 of TEA was predicted to be almost the same as the PEA case. The optimized lattice angles as well as the observed dspacing of TEA2SnI4 supercell were found to be substantially different from other PEA series as being shown in Table S5. Consequently, such distinct intermolecular packing of TEA cations introduced different-but-beneficial cell strain to its inorganic counterpart. In Figure 4, the partial density of state for the p band of iodide in TEA2SnI4 was found to be more delocalized and closer to the Fermi level than that of pFPEA2SnI4 and PEA2SnI4, which could contribute a higher 𝜀𝑤 value for TEA2SnI4.


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Journal of the American Chemical Society proportional to the inverse of the exciton binding energy for quantum wells.47 Similar phenomenon was also observed in other 2D perovskite systems,48 in which the radiative lifetime decreases as the exciton binding energy increases. In theory, the increase of exciton binding energy should facilitate the electron-hole recombination rate and hence decrease the emission radiative lifetime. This, in combination with the decrease of the nonradiative decay rate due to the increase of binding energy, i.e., decrease of the defect sites, leads to a trend of increasing PLQY upon increasing the dielectric contrast (εw / εb)2 (eq. (1)). Further firm support is given by a linear plot of PLQY as a function of (εw / εb)2 calculated for the studied perovskite nano-disks (see Figure 5c). The combination of experimental and theoretical approaches thus demonstrates that the photophysical properties of tin perovskite nano-disks can be harnessed through modulation of dielectric confinement using different aromatic organic cations. Figure 3. Calculated dielectric profiles of tin-iodide perovskite nano-disks. Green, orange, and red lines represent PEA2SnI4, p-FPEA2SnI4 and TEA2SnI4, respectively. The Y axis represents the dielectric constants of cation layers and inorganic layers under the frequency corresponding to emission wavelengths individually. The upper figure is the crystal structure of TEA2SnI4, where blue and green region represent quantum well (Sn-I layer) and barrier (organic layer) region, respectively.

Figure 4. The calculated partial density of states for p bands of Sn, I of TEA2SnI4, p-FPEA2SnI4 and PEA2SnI4 supercells, respectively. The p bands of Sn were magnified by 5 times.

Summing up the above computational approach, the strength of dielectric confinement, which is proportional to (εw / εb), is predicted to follow the order of TEA2SnI4 > p-FPEA2SnI4 > PEA2SnI4. This correlates well with the experimental result of most red-shifted emission wavelength being ascribed to the TEA2SnI4 nano-disks (see eq. (2)). As a result, the optical band gap as a function of the ratio of dielectric constant (εw / εb) exhibits a linear behavior (see Figure 5a), which decreased as increasing (εw / εb), supporting the relationship expressed in eq. (2). The plot of radiative lifetime r (1/kr) versus (εb / εw)2 (Figure 5b) illustrates a linear behavior; that is the decrease of (εb / εw)2 results in a decrease of r. In other words, the increase of binding energy gives rise to a shorter radiative lifetime. This result may not be surprising by knowing the relationship between the exciton binding energy and radiative lifetime. Feldmann has reported that the radiative lifetime is

Figure 5. a) The correlation plot of band gaps of tin perovskite nano-disks versus εw / εb. Here, the red line represents linear regression with the square of the sample correlation coefficient r2 = 0.99991. b) The correlation plot of exciton radiative lifetime versus (εb / εw)2. The linear regression possesses r2 = 0.99829. c) The correlation plot of PLQY versus (εw / εb)2. d) Photostability test of TEA2SnI4 nano-disks in degassed toluene with 0.1 volume% pentanoic acid solution under continuous 375 nm (10 or 150 mW cm−2) illumination.

Last but not the least, the stability of the TEA2SnI4 nano-disks formed in the presence of 0.1 vol% pentanoic acid is significantly improved in comparison to the previously reported BAOASnI4 (BA: butylammonium and OA: octylammonium) and PEA2SnI4 nano-disks.35,39 As shown in Figure S7, the results showed that the emission intensity of the BAOASnI4 nano-disks decreased to 20% of its original value after irradiation with 375 nm light (10 mW/cm2) for 2 hours. In comparison, under an identical experimental condition, the aromatic ammonium tin perovskite nano-disks demonstrated significant improvement in their photostability. Moreover, no noticeable decay of the photoluminescence intensity of TEA2SnI4 was observed upon irradiation of the nano-disk toluene suspension for 120 hours. This better



photostability could be rationalized by the tight packing alone the (001) direction for the TEA2SnI4 nano-disks, resulting in a lower defect density and hence a protective shield to avoid Sn (II) oxidation. To further test the photostability of TEA2SnI4, upon irradiation with high intensity (150 mW/cm2), the result (Figure 5d) showed that the emission intensity of nano-disks decreased to 60% of its original value after irradiation for 120 hours. This degrade may arise from the heat damage to nanodisks by high intensity irradiation, leading to more defect formation within lattice. CONCLUSION In summary, the mismatch of dielectric constant of inorganic frameworks and organic cations was found to play a crucial role in the photoluminescent properties of aromatic ammonium tin perovskite nano-disks, such as emission peak position, exciton relaxation dynamics and PLQY values. The correlations among these photoluminescent properties and dielectric confinement effect were then established semi-empirically. The enhancement of dielectric confinement leads to a red shift of the emission peak wavelength, a shortening of the exciton recombination time and an increase of PLQY. Upon further fine-tuning with pentanoic acid as the additive, TEA2SnI4, which possesses the largest dielectric contrast between organic ammonium cation and semiconducting tin iodide layer in this study, shows a record high PLQY value of 21%, one of the highest achieved to date for tin halide perovskite nanomaterials with direct band narrow emissions.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Additional synthesis, characterization data, and computational details are provided.

AUTHOR INFORMATION Corresponding Author *Hao Ming Chen, E-mail: [email protected] *Ming-Kang Tsai, E-mail: [email protected] *Pi-Tai Chou, E-mail: [email protected] *Ching-Wen Chiu, E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the Ministry of Science and Technology of Taiwan (MOST 107-2113-M-002-007; MOST 107-2119-M-002-002; MOST 107-2113-M-003-007) and National Taiwan University (NTU-108L880104). Computational resources are supported by the National Center for HighPerformance Computing of Taiwan and the Center for Cloud Computing in National Taiwan Normal University.

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Harnessing Dielectric Confinement on Tin Perovskites to Achieve

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